Background/Purpose:

Systemic sclerosis (SSc) is characterized by uncontrolled activation of fibroblasts resulting in tissue fibrosis in which the TGFβ signaling plays a main role. Signal transducer and activator of transcription 3 (STAT3) is a transcription factor belonging to the family of seven proteins transmitting the signals from plasma membrane receptors to nucleus with subsequence modulation of gene transcriptions involved in angiogenesis, immune responses and metastasis. Nuclear translocation occurs upon phosphorylation by several receptors tyrosine kinases, particularly by Janus kinase 2 (JAK2). In the present study, we evaluated the role of STAT3 as a downstream mediator of TGFβ signaling pathway in the pathogenesis of SSc and analyzed the potential of STAT3 inhibition as a novel anti-fibrotic approach.

Methods: Activation of STAT3 in the human skin and in murine models was analyzed by IF staining for phosphorylated and thereby activated STAT3 (pSTAT3). Selective inhibitors of JAK2 and STAT3 and knockdown strategies were used to interfere with JAK2 and STAT3 signaling in vitro and in vivo. The anti-fibrotic potential of STAT3 inhibition was evaluated in two mouse models of SSc: bleomycin-induced fibrosis and fibrosis induced by overexpression of a constitutively active TGFβ receptor type I (TBR).

Results: Increased activation STAT3 signaling with accumulation of pSTAT3 in fibroblasts was observed in the skin of SSc patients and in murine models of SSc. Stimulation with TGFβ increased the expression of STAT3 protein and induced nuclear accumulation of pSTAT3 in cultured fibroblasts. Pre-incubation with specific JAK2 inhibitor abrogated the TGFβ induced induction of STAT3 as well as nuclear accumulation of pSTAT3, demonstrating that TGFβ activates STAT3 in a JAK2 dependent manner. Inactivation of STAT3 with the selective STAT3 inhibitor S3I-201 significantly reduced the stimulatory effects of TGFβ on the protein collagen level (-56 %, p=0.05) and the mRNA level of Col1a1 (-71 %, p=0.0095) and Col1a2 (-35 %, p=0.0095) and myofibroblast differentiation in cultured human fibroblasts. The same results were observed when STAT3 was inactivated by conditional knockout in murine fibroblasts. Moreover, treatment with S3I-201 (10 mg/kg) exerted potent anti-fibrotic effects in bleomycin- and TBR induced fibrosis. In the model of bleomycin induced fibrosis, treatment with the specific STAT3 inhibitor decreased dermal thickening by 33% (p=0.0009), hydroxyproline (HP) content by 51 % (p=0.001) and myofibroblast counts by 55 % (p=0.0009) Potent anti-fibrotic effects with reduced dermal thickening, decreased HP content and reduced myofibroblast counts were also observed in TBR induced fibrosis.

Conclusion: We characterize STAT3 as a novel downstream mediator of the profibrotic effects of TGFβ in SSc. We demonstrate that TGFβ activates STAT3 and that this activation is required for the profibrotic effects of TGFβ. Inhibition of STAT3 prevents fibroblast activation and shows potent antifibrotic effect in different preclinical models of SSc. These findings may have direct translational implications considering that several STAT3 inhibitors are currently in clinical trails.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/activation-of-the-signal-transducer-and-activator-of-transcription-3-by-transforming-growth-factor-beta-promotes-fibroblast-activation-and-tissue-fibrosis/